Conventionally, image scanning apparatuses such as an image scanner, copying machine, and the like use a contact image sensor (to be abbreviated as CIS hereinafter) which scans a document in proximity to it at an equal scale as an image sensor which optically scans image information of a document and converts it into an electrical signal.
FIG. 8 depicts a sectional view showing an example of the arrangement of a conventional color CIS unit.
Light emitted from a light source 71, which is arranged on the end face of a light guide member 72 having a shape elongated in the main scan direction, enters the light guide member 72 and is guided in the longitudinal direction thereof, and almost uniformly illuminates a document 79 placed on a transparent document support table 78 in a line pattern in the main scan direction. This light source 71 comprises light-emitting elements R, G, and B which respectively have three emission colors red, green, and blue (to be abbreviated as R, G, and B hereinafter). In FIG. 8, a frame 73 fixes constituting members such as the light guide member 72, a lens array 74, a sensor board 76, and the like at predetermined positions.
Note that three-color, that is, R, G, and B LEDs are used as these light-emitting elements, which time-divisionally undergo light-ON driving independently of R, G, and B under the control of a light source control circuit (not shown). That is, the light-ON time period of each color LED is ⅓ of one light-ON period. Reflected light from the document 79 illuminated in this way is focused by a lens array 74, and is guided to photoelectric conversion elements 75 arranged on the board 76. An electrical signal photoelectrically converted by the photoelectric conversion elements 75 is externally output via a connector 77. The photoelectric conversion elements 75 as many as the number of pixels to be scanned are arranged along the main scan direction of the document. For example, upon scanning an A4-sized document at 600 dpi, about 5,000 photoelectric conversion elements are arranged.
By contrast, patent reference 1 proposes a method of irradiating with light emitted by LEDs of two colors by turning off an LED of one color of the three color LED light sources. This method is called “complementary color scan”, and has an advantage of increasing the amount of irradiated light compared to the prior art. Patent reference 2 describes a technique that adopts a color separation scheme in which three arrays of photoelectric conversion elements are arranged on a sensor IC using a color CIS, and R, G, and B color filters are mounted on respective element arrays. With the technique of patent reference 2, a white light source such as an Xe lamp, white LED, or the like is used as the light source, and that white light source is kept ON. Hence, the light-ON time period of the light source is three times longer than the prior art.
Furthermore, patent reference 3 describes a technique that simultaneously detects signals of R, G, and B, three colors by a single light-receiving unit. A feature of this technique is to exploit entry length differences of light in silicon for color separation. With this technique, a triple well structure is formed in a silicon substrate, and light rays of different colors (400 to 490 nm, 490 to 575 nm, and 575 to 700 nm) are detected by detection units located at different depth positions. As a result, respective pieces of information based on three primary colors (R, G, and B) can be measured at an identical position.
Patent reference 1: Japanese Patent Laid-Open No. 5-122455 (FIG. 2(a))
Patent reference 2: Japanese Patent Laid-Open No. 2003-32437
Patent reference 3: Japanese Patent Laid-Open No. 2002-513145 (U.S. Pat. No. 5,965,875)